Add support of exporting project labels

It will be grate if gcppromd could export project labels list. 
Let me explain usage: 

We are using one project per product and project labels to hold all meta about virtual machines in it. 
Adding project level labels will remove duplication for such labels as 
* product_name (project_code), 
* product_env_type (prod|rc|dev)
and other common labels for project